The present invention relates generally to systems for cancelling distortion caused by nonlinearities in the transfer functions of electronic devices by generating complementary distortions and, more specifically, to a system for cancelling both odd and even order distortions by generating complementary odd and even order distortions.
The nonlinearities inherent in many opto-electronic devices produces distortion in optical communication systems. Linearization circuits known in the art combine the device signal with a signal that is equal in magnitude but opposite in sign, such that distortion is canceled. In U.S. Pat. No. 5,227,736, issued to Tucker et al. a linearization circuit cancels second-order nonlinearities. Two 180 degree hybrids and two linear amplifiers are used to generate distortion. The isolated port of the second hybrid produces the distortion signal. The other output of the second hybrid produces a signal that includes the fundamental signal. The amplifiers must be linear to avoid generating excessive odd-order distortion along with the fundamental signal.
In U.S. Pat. No. 4,992,754, issued to Blauvelt et al. a portion of the main signal is split off into a distortion generator that produces second-order distortions. These distortions are adjusted in phase and amplitude and recombined with the main path signal. A time delay is included in the main signal path to equalize delays in the paths. The composite signal is then fed to the device to be linearized.
In U.S. Pat. No. 5,132,639, issued to Blauvelt et al., two portions of the main signal are split off into two distortion generators. One distortion generator generates second-order distortions in response to the portion of the main signal it receives. The other distortion generator generates third-order distortions in response to the portion of the main signal it receives. These distortions are adjusted in phase and amplitude and recombined with the main path signal. A three-way directional coupler combines the signal of the main path with those of the two distortion generator paths. A time delay is included in the main signal path to equalize delays in the paths. Although this device cancels second and third-order distortions, it uses two distortion generators.
It would be desirable to cancel both odd and even-order distortions using a distortion generator having only a single section. Such a device would be more economical to produce and maintain than those known in the art because the part count and complexity would be reduced. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.